1. Field of the Invention
The present invention relates to a fluid sensing device and more particularly to an optical-electrical liquid probe which is intended to determine the presence or absence of a liquid in the immediate vicinity of the probe, by an optical method, and to signal it electrically to a position remote from the probe.
2. Description of the Prior Art
There has been proposed an optical-electrical liquid probe comprising a light emitter, a light receiver, constructed as a photoelectric transducer, and a body which is translucent to light rays and which has at least one surface which can be exposed to a liquid. The light emitter sends the light rays, which it produces, into the translucent body, so that they strike, from within the body, the surface which can be exposed to liquid, and are refracted in the absence of liquid at the said surface, in the direction of the light receiver. If, in the case of such a probe, the surface which can be subjected to liquid adjoins a liquid, the light rays issuing from this surface suffer no refraction, or at least a different refraction than if liquid is absent, the result of which is that virtually no light rays strike the light receiver and this results in a change in the electrical state of the light receiver. This previously proposed probe reacts in exactly the same manner whether the liquid present is translucent or opaque to light. Further, if an operating fault occurs, for example an interruption in the current supply, or the failure of the light emitter, the electrical output of the light receiver is in the same state as occurs in the presence of a liquid on the surface of the translucent body. Hence the use of this previously proposed probe offers particularly high safety in ensuring that containers are not over-filled.
An object of the present invention is to provide a liquid probe of the above type which remains operational even after exposure to a relatively highly viscous liquid and which is substantially insensitive to extraneous light and/or scattered light, that is to say to light which either does not originate from the light emitter fitted into the apparatus or which is scattered by particles contained in the liquid.